Algae, especially microalgae, are a group of organisms that have received a great deal of interest during the current energy and fuel crisis, due in part to their relatively low cost to large biomass ratio. For example, it is believed that algae can produce as many as 30 times more biomass than land crops, per acre. Further to this, most algae can live in conditions having only water (untreated), a carbon source, and sunlight, as they produce their own energy for metabolism. Furthermore, because the fatty acid of algae can be extracted and used to generate biofuel (among other natural products and bioactive compounds) algae has been generally considered a renewable energy source with tremendous potential. Besides potential in biofuel, microalgae can be used to produce a variety of nutriceuticals including unsaturated fatty acid, pigment, antioxidant, surfactant, and others. In addition, genetically modified microalgae can be used to produce an even broader range of products, including therapeutic proteins, bioplastics, surfactant, terpenoid products, and others.
Algae can be grown in a variety of systems. For example, algae can be grown in both open-culture systems, such as ponds, lakes and raceways. The advantage of such systems is that they are generally cost-effective and easy to manage. Algae can also be grown in highly controlled closed-culture systems, similar to those used in commercial fermentation processes. However, closed-culture systems typically command higher investments and operating costs, but are independent of all variations in agro-climatic conditions and are very closely controlled for optimal performance and quality.
One of the major technical barriers for algae biofuel production is the considerable harvesting and extraction cost associated with the process for the separation of Single Cell Oleaginous (SCO) microalgae from liquid medium. For example, algae harvesting and lipid extraction steps alone can account for up to 50% of the total cost of algal biofuel production.
Current harvesting strategies include centrifugation, column flotation cross-flow microfiltration, electrolytic flocculation, and other such methods. However, all of these strategies have limitations when scaled up for commercialization. For example, the centrifugation of millions of gallons of liquid medium requires considerable energy input and prohibitively high production costs. Thus, the development of an effective harvesting strategy is crucial to reduce the cost of algae biofuel (Singh et al., 2010, Renew Sust Energ Rev 14(9): 2596-2610). The ability to integrate low cost harvesting with high yielding cultivation would therefore enable an economically viable algal biofuel production system. In addition, such technology can also reduce the cost for algae-based production of nutriceuticals like unsaturated fatty acids, antioxidants, surfactant, therapeutics, bioplastics, proteins, and other chemical products.
Filamentous fungi have been broadly used for the industrial production of various proteins, lipids and other therapeutics. For example, filamentous fungi like Mortierella isabellina, Mucor circinelloides and Cunninghamella echinulata have been used for SCO lipid production. In submerged cultivations, fungi can generally grow into two different morphologies, filamentous and pellet morphology, depending on culture conditions and medium compositions. The pellet morphology, as compared to filamentous morphology, can significantly decrease viscosity and enhance mixing and mass transfer, which can in turn greatly improve fermentation performance (Vansuijdam et al., 1980, Eur J Appl Microbial Biotechnol 10(3): 211-221). For example, Hiruta et al. reported that pelletization can lead to high yields of gamma-linolenic acid (GLA) by Mortierella ramanniana (Hiruta et al. 1997, J Ferment Bioeng 83(1): 79-86).
Despite the various advantages, the pelletization process is difficult to effectively control. For example, oxygen, pH and other factors can all impact the formation and size of the pellets as well as the fermentation performance. That said, attempts to optimize fungal pelletization for industrial applications have been made (Verma et al. 2009, J Pure & Appl Microbial 3(2): 559-565). Despite these attempts, pelletization has seldom been used for improving the fermentation performance in a co-cultivation setting, due in part to the challenges in optimizing a more complex system. In particular, pelletization has never been explored for green algae cultivation and processing. The pelletization of algae-fungi co-cultivation has never been achieved prior to the present invention.
Efficient production of biofuels from green algae would have significant implications in a wide range of industrial applications. However, the inability to effectively harvest such algae derived biofuels in a commercial setting has thus far limited its use. Therefore, a need exists for the development of a culturing system and method which provides for a more simplified harvest and increased yields. The present invention addresses this unmet need in the art.